This application claims the priority of Korean Patent Application No. 2003-23213 filed on Apr. 12, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a memory element, and more particularly, to a phase-change memory element utilizing characteristics of the phase change of materials and to a method of fabricating the same.
2. Description of Related Art
A phase-change memory device stores information utilizing the electrical conductivity difference between a crystalline phase and an amorphous phase of a specific material. A phase-change memory device is formed with a semiconductor substrate which is equipped with a transistor element or a diode element for addressing and a read/write operation, and these elements are electrically connected with a phase-change region. Because information is stored utilizing the conductivity difference from a phase-change, data is stored as a characteristic of the phase-change memory element including the phase-change region.
Looking into the operation of phase-change memory device, a current flow through a transistor or a diode heats the phase-change region. Then, by a reversible transformation of the structure of the phase-change material between a crystalline phase and an amorphous phase, information is stored, and then the stored information is read by detecting the resistance of the phase change material by a relatively low current flow.
Up to now, the most difficult technical problem for the embodiment of the phase-change memory element has been the excessive power consumption for a writing operation, especially, when there is a transformation from a crystalline phase to an amorphous phase; that is, a reset operation requires heating the phase-change material beyond the melting point. The most serious problem in this matter is that the integration capability of the phase-change memory is confined in association with the fact that the smaller the size of transistor element for the action of phase-change element, the smaller the power transmission through the transistor element to the phase-change memory element.
In order to have the phase-change memory element operating with a low current, it is necessary to reduce the volume of the phase-change region of the element as much as possible, and to improve the thermal isolation of the phase-change region. In this direction, various structures of a phase-change element for operating with a low current have been studied.
For example, according to U.S. Pat. No. 5,933,365 (“Memory element with energy control mechanism”, by Patric Klersy et al, registered on Aug. 3, 1999), a lower electrode used as the heating layer contacts a memory layer, that is, a phase-change layer within a contact hole. Accordingly, the phase-change layer contacts the lower electrode, practically, at the bottom of the contact hole, and a phase change takes place on such contacting surface. In this structure of an element, it is possible to reduce the volume of the phase-change field but the volume of the phase-change field is ultimately limited by the resolution of the photo lithography process which is under gone to form the contact hole. Therefore, for a highly integrated memory element, the power that is consumed could still remain as a problem.
Also, in U.S. Pat. No. 5,687,112 (“Multibit single cell memory element having tapered contact”, by Stanford R. Ovshinky et al, registered on Nov. 11, 1997), a structure for reducing the phase-change field was suggested. According to them, a tapered contact tip is prepared, and the tip of the tapered contact tip is arranged to contact the memory layer and the phase-change layer. In spite of this approach, since the volume of the phase-change field could change excessively even with a minute variation in the contact surface, securing the uniformity of the element's characteristics could be very difficult.
Therefore, the development of a phase-change memory element having a stabilized structure which secures the uniformity and reproducibility for a manufacturing process and is capable of operation with a low power is still required.